A cable carrying high voltages must be designed to operate without voltage breakdown stress in the cable insulation and without corona externally of the cable insulation.
The possibility of breakdown stress of the insulation occurs at the surface adjacent the conductor. Increasing the conductor diameter reduces the voltage stress applied to the insulation adjacent the conductor. However, the cost of the cable increases with the diameter of the conductor. It is also expensive but desirable to utilize a conductor as cylindrical as possible so that its effective diameter predictably operates without undue voltage stress concentrations. One economizing approach is to fabricate a conductor of stranded leads and extrude thereover a jacket of semiconductive, or high resistance, dielectric material such as carbon impregnated polyethylene. This results in a composite conductor of inexpensive material having an even cylindrical shape and relatively large diameter. Insulation can then be intimately bonded to the composite conductor to exclude deleterious air pockets.
Elimination of corona externally of the cable has been heretofore prevented by fabricating the cable insulation to a relatively large diameter. Such practice has resulted in very stiff cable having expensive and consequently expensive amounts of insulation. Reducing the insulation diameter by using a more effective dielectric has not been satisfactory due to increased costs of the better dielectric and its inherent stiffness.
Another drawback of solid insulated high voltage cable is a requirement to exclude air. Any damage to the cable insulation allows air leakage toward the conductor, increasing the chance of corona externally of the cable. Thus protection of the cable insulation has heretofore been required by providing a tough jacket thereover, further adding to stiffness.